You can still use selenium for IE Explorer. I don't know if there is a plugin, but that shouldn't stop you. You'll get a lot of benifit from using the same tool to test on all browsers, so I would think you would be better served to learn how to use selenium with IE Explorer rather than trying to re-implement all your tests in another tool

TestPlan can use the Selenium backend and works with both FireFox and Internet Explorer. It can also work with a display-less browser (HTMLUnit) so you can code your basic tests quickly and not suffer the loading overhead of the browser. Once it basically works you test it against the browser.